Since a chlorosulfonated ethylene-.alpha.-olefin copolymer is excellent in dynamic characteristics and low-temperature resistance as compared with chlorosulfonated polyethylene using a high-density polyethylene, the application of the copolymer to dynamic uses such as motorcar belts, etc., has been investigated.
One example of the dynamic uses to which the application of the chlorosulfonated ethylene-.alpha.-olefin composition has been investigated is joint boots for motorcars. For example, a uniform joint for smoothly transmitting rotation between the shaft of the driving shaft and the shaft of the side being driven is provided to a drive shaft of a front-wheel-drive car, and a uniform joint boot for enclosing a grease in the inside thereof is mounted in the form of covering the portion.
Hitherto, as the material for the joint boot, a chloroprence rubber has been used. However, with increase of the performance and improvement of the durability of recent motorcars, higher durability is required for the joint boot. In particular, since there is a limit about ozone resistance and heat resistance of a chloroprene rubber, a material more excellent in the ozone resistance and the heat resistance has been demanded. For this purpose, recently, an attempt of using a chlorosulfonated ethylene-.alpha.-olefin copolymer more excellent in the ozone resistance and the heat resistance than the chloroprene rubber as the material for a joint boot has been investigated.
For obtaining a vulcanization property suitable for each use, the chlorosulfonated ethylene-.alpha.-olefin composition is usually used as a composition thereof with various kinds of additives such as a vulcanizing agent, a vulcanization aid, a vulcanization accelerator, a reinforcing agent, a plasticizer, a processing aid, an antioxidant, etc. JP-A-1-210443 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a chlorosulfonated ethylene-.alpha.-olefin copolymer composition which is excellent in the heat resistance, the cold resistance, and the water resistance, and is useful in the field of rubber belts, prepared by compounding 100 parts by weight of a chlorosulfonated ethylene-.alpha.-olefin composition containing from 25 to 32% by weight of chlorine and from 0.3 to 2.5% by weight of sulfur with from 10 to 35 parts by weight of hydrotalcites, from 0.5 to 5 parts by weight of thiurams, from 0.5 to 5 parts by weight of maleimides, and from 0.2 to 4 parts by weight of dithiocarbamates.
However, a joint boot is a part which receives repeatedly a relatively large deformation under use conditions as compared with a rubber belt. Thus the joint boot is required to have a fatigue resistance capable of enduring the repeated deformation. Also, supposing the use in a cold district, the joint boot is required to have tensile characteristics capable of enduring a large deformation at low temperature. Further, the joint boot using the chlorosulfonated ethylene-.alpha.-olefin copolymer composition as proposed in JP-A-1-210443 described above is yet insufficient in the tensile characteristics and the fatigue resistance at low temperature. Therefore, the joint boot has not yet practically be used at present.